In addition to the knife, oxygen, vision mask,
and flippers, some missions require other devices to assure the
success of the project and the safe return of the man himself. Some
missions, such as mapping projects, require too that accurate
knowledge of the bottom, direction of currents, and depths
encountered, be recorded. This knowledge is obtained partly from past
experience but mainly from small, compact, accurate instruments
carried by the Frogmen. Here we see a warm-water swimmer about to
start his project, so equipped. On his right wrist he wears an
accurate magnetic compass that can be read easily under water. On his
left wrist he wears, first, the conventional waterproof wrist watch
next to his hand and then above it up his arm an accurate depth gauge
to tell him just how far below the surface he is. These three, plus
visual observation, will enable him to make a quite accurate map of
his underwater terrain. Hanging about his neck is a sheet of plastic
and a sharp-pointed stylus. With the latter , notes can be made upon
the plastic sheet (in place of paper and pencil), from which to make
the accurate map later on.